Examples on Beams.
Example 25. Find the safe concentrated load in the following-
cases by the approximate formula.
(1) Wrought Iron Plate Girder. Each flange, 10" x J" ; angles?
3 i" x 3i" x i" ; total depth, 3 ft. ; span, 28 ft. ; /t or/c = 5 tons.
Allow for |" rivets.
(2) Wrought Iron Rolled Cantilever, H Section. Each flange,
4i" x f"; depth, 8" ; overhang, 8 ft. ; ft or/c=* 5 tons.
(3) Cast Iron Girder. One flange, 3"xr|"; one flange, 9"xiJ";
depth, 12"; span, 20 feet.
"(All from Eng. Exam. 1891 and 1892.)
/ \ iTtr
(2) W = - -__
(3)/c <3C = 3 x ii x 4= 18 tons ; and/t at=9 x i J x i J= 17 tons
, 4x17/2 4x17x10^-
.-. W = 3 ' =1 - i - ^tons = 2-97 tons.
/ 20X12 - ZŁ- -
Example 26. Find the depth of an engine guide bar 10" wide
and 4 ft. span. Total piston pressure = 25 tons ; length of connecting
rod = twice stroke ; and greatest obliquity supposed to occur with
guide block at centre of span. /0=5 tons.
(Hons. Mach. Constr. Ex. 1892.)
Draw crank and rod to scale, Fig. 407. Then the forces are as
at A, and the triangle of forces is drawn parallel, as at D.
_. E press, on bars ., . 1x25.
But = * : - and press, on bars = - - = 6*25 tons
D piston press. 4
4X10X5 -
Example 27. The girder stays of a combustion chamber are 21
span, and are spaced 8J" centres apart (see Figs. 31 1 and 312), the sec-
tion being rectangular, 5!" deep by if" wide. There are two bolts to
each stay, 7" apart (Fig. 408). Find the greatest stress in the stay when
steam pressure = 225, Ibs, per sq. in. (Hons. Mach. Constr. Ex. 1891.)
The roof plate is equivalent to a continuous girder over three
spans, as at p. 445, so the pull on each bolt would apparently
be ^J- W. But the plate is continuous transversely also. Therefore